In general, a shock absorber is to damp vibration of a vehicle, equipment, a construction and the like. A hydraulic shock absorber generating a damping force using resistance of an operating liquid made of oil, water, an aqueous solution or the like includes a cylinder in which the operating liquid is filled therein and a liquid chamber is formed, an annular rod guide fixed to the cylinder and closing one side of the liquid chamber, a piston rod penetrating the rod guide and going into/out of the cylinder, a piston held by the piston rod and dividing the liquid chamber into an extension-side chamber and a compression-side chamber, a piston passage formed in the piston and allowing the extension-side chamber and the compression-side chamber to communicate with each other, and a damping valve giving resistance to a flow of the operating fluid passing through the piston passage.
The above-described hydraulic shock absorber is configured such that, in extension/contraction of the piston rod going into/out of the cylinder, the operating liquid in one of the chambers pressurized by the piston passes through the piston passage and moves to the other chamber, whereby the damping force caused by the resistance of the damping valve is generated (see JP2011-174501A, JP2012-167785A, JP2010-185571A, for example).